tailieunhanh - Ideas of Quantum Chemistry P33

Ideas of Quantum Chemistry P33 shows how quantum mechanics is applied to chemistry to give it a theoretical foundation. The structure of the book (a TREE-form) emphasizes the logical relationships between various topics, facts and methods. It shows the reader which parts of the text are needed for understanding specific aspects of the subject matter. Interspersed throughout the text are short biographies of key scientists and their contributions to the development of the field. | 286 7. Motion of Nuclei equation for every configuration R of the nuclei and take the eigenvalue . an analogue of E0 R as V R . This would take so much time even for small systems composed of a few atoms that maybe even after some cursing we would abandon this method with a great feeling of relief. Even if such a calculation required huge computation time it would give results which would have been quite simple in their overall features assuming that the molecule has a pattern of chemical bonds . It just would turn out that V would be characterized by the following Chemical bonds. V R would be obtained about its minimum if any chemical bond between atoms X and Y had a certain characteristic reference length r0 that would depend on the chemical species of the atoms X and Y. If the bond length were changed shortened or elongated to a certain value r the energy would increase first according to the harmonic law with force constant kXY and then some deviations from the harmonic approximation A harmonic term of the kind 2kXY r - r0 2 incorporated additively into V replaces the true anhar-monic dependence by a harmonic approximation assumption of small amplitudes as if the two atoms had been bound by a harmonic spring in the formula the atomic indices at symbols of distances have been omitted . The most important feature is that the same formula 2kXY r - r0 2 is used for all chemical bonds X-Y independently of some particular chemical neighbourhood of a given X-Y bond. For example one assumes that a non-distorted single C-C bond19 has a characteristic reference length r0 A and a characteristic force constant kXY 317 Jo A similarly some distinct parameters pertain to the C C bond r0 A kXY 690 21 molA2 Bond angles. While preserving the distances r in the A-B and B-C bonds we may change the bond angle a A-B-C in this way changing the distance. A corresponding change of V has to be associated with such a change. The energy has to .